It is known that silicon can be made in rod form by a process referred to as the Siemens process. A mixture comprising hydrogen and silane (SiH4) or a mixture comprising hydrogen and trichlorosilane (HSiCl3) is fed to a decomposition reactor containing, seed rods which are kept at a temperature of more than 1000° C. Silicon is deposited on the seed rods and by-product gas mixtures exit in a vent stream. When a mixture comprising hydrogen and trichlorosilane is used, the vent stream may include hydrogen, hydrogen chloride, chlorosilanes, silane, and silicon powder. For purposes of this application, the term ‘chlorosilanes’ refers to any silane species having one or more chlorine atoms bonded to silicon and includes, but is not limited to monochlorosilane (H3SiCl), dichlorosilane (H2SiCl2), trichlorosilane (HSiCl3), tetrachlorosilane (SiCl4), and various chlorinated disilanes such as hexachlorodisilane and pentachlorodisilane. For purposes of this application, the term ‘silicon monomer’ refers to any silane species having one silicon atom per molecule (e.g., silane, or HSiCl3, or a combination of HSiCl3 and SiCl4). In the vent stream, hydrogen and chlorosilanes such as SiCl4 and HSiCl3 may be present both from un-reacted feed gas and reaction product from the decomposition. The vent stream is passed through a complex recovery process where condensations, scrubbing, absorption and adsorption are unit operations often used to facilitate the capture of feed material HSiCl3 and hydrogen for recycle. One problem associated with the Siemens process is that it is difficult to achieve a high yield of polycrystalline silicon product to silicon fed due to the chemical equilibria and kinetics that control this reaction process.
Quite often only 50%, or less, of the maximum theoretical yield of polycrystalline silicon is achieved. Furthermore, the Siemens process requires relatively high energy input to achieve this relatively low yield.
An alternate process is to feed the mixture comprising hydrogen and silane or the mixture comprising hydrogen and trichlorosilane to a fluidized bed containing silicon nearly spherical beads that are maintained also at high temperature. The beads grow in size, and when large enough, are passed out the bottom of the fluidized bed reactor (FBR) as product. The vent gases exit the top of the FBR and are sent through a recovery process similar to the one described above for the Siemens process. Yield in this process may be nearly 90% of theoretical maximum, as compared to the 50% to 70% for the Siemens process.
One problem with the FBR process is that the beads must be heated to a temperature higher than the average bed temperature to facilitate heat transfer. That can be done, for example, by use of a hot walled reactor, microwave energy, radio frequency inductive heating, or infrared radiation. All heating methods have unique operating problems. One problem, however, is that the bottom of the FBR may be hot, and the feed gas is reactive when it contains only HSiCl3 and hydrogen. As a result, the feed gas distributor, clusters of large beads, and reactor side walls are prone to rapid deposition of silicon. Those deposits subsequently disrupt the proper feed distribution, product separation, and heat transfer of the system. Another problem with the FBR process is the product quality is generally insufficient for use in integrated circuit manufacture; however, the product of the FBR process may be used in solar grade applications.
There is a need in the polycrystalline silicon industry to improve efficiency of polycrystalline silicon production with Siemens reactors to reduce by-products and energy consumption. There is a need in the polycrystalline silicon industry to improve FBR technology to prevent silicon deposits from forming on the walls of the FBR.